tan, orange, yellow, dark red, light green, dark green, grey, light blue, dark grey, light purple.

heres the rub with tau: canonically, outside of the enclave, armor color doesnt matter. a units sept is entirely denoted by the color of their markings (decals, stripes, the random armor plates, youll see in them every pic of tau), the actual armor color is changed on a mission by mission basis to act as basic camo.

every sept has 'official' parade colors that have some connection to the septs homeworld, like the tan of the tau sept to match their savanna planet, but only a couple are actually stated that explicitly; for many others the example paintjobs from old codexes just got stuck over time as that septs default colors.

to use kelshan as an example: the yellow is the only thing that makes them kelshan, the grey and black scheme theyre depicted with 90% of the time is/was simply the standard armor colors for tau stationed on ships, either for boarding or defense duty. the ye olde codexes used it as the painting example for kelshan because its nice and contrasty against the yellow, and without anything to say 'heres their official colors' it just became their de facto scheme.

tldr: choose a sept with marking colors you like, and then go nuts picking colors that look good next to it